Thermionic conversion means



United States Patent O THERMEONIC CONVERSION MEANS Russell G. Meyerand,Jr., and Donald W. Bell, Glastonbury, and Robert H. Bullis, WestHartford, Conn.,.as signers to United Aircraft Corporation, EastHartford, Conn., a corporation of Delaware Filed .lune 22, 1964, Ser.No. 376,728

14 Claims. (Cl. 310-4) This invention relates to thermionic convertersand more particularly to apparatus and method for increasing theperformance and/ or reducing the weight of thermionic systems byreducing the net thermal radiation between the emitter and the collectorof a thermionic converter, that is, by reducing7 the radiated that isabsorbed by the collector assembly and has to subsequently 'be rejectedfrom the system at the collector assembly temperature.

A thermionic converter is a device in which an electron emitter,operating in alkali metal or other easily ionizable environment at atemperature of about 1400 K., produces electrons by thermionic emissionand these electrons so emitted pass over to a cooler, approximately 900K., electron collector. The electrons so collected are then passedthrough electrical leads through an electrical load back to the emitter.In this fashion, thermal energy is converted into electrical energywhich acts upon the load to produce work. The output voltage of thedevice is irnproved by maintaining the work function of the collector ata lower value than that of the emitter.

The proximity of the emitter or cathode to the collector or anode causesthe thermal radiation, primarily infrared (micron wave lengths), fromthe emitter to be absorbed in t-he collector of conventional thermionicconverters, thereby increasing the temperature of the collector assemblyand requiring excessive cooling to maintain the collector at the desiredoperating temperature.

It is an object of this invention to teach methods and apparatus toreduce the amount of thermal energy radiated by the emitter which isabsorbed by the collector assembly and thereby reduce the total amountof thermal energy which has to be rejected from the thermionicconversion system at the collector assembly temperature. Our teaching isto either use a thermal radiation reflecting collector which willreflect the emitter generated thermal radiation back to the emitter, orto use a thermal radiation transparent collector which will directlyreject, due to its transparency, the emitter -generated thermal energy.Ideally, the simplest approach to reducing the amount of emitter thermalradiation absorbed by the collector assembly is to use a low emissivitycollector surface to reflect the thermal radiation from the emitter backto the emitter there-by reducing the amount of thermal energy absorbedby the collector assembly and also increasing the overall eiciency ofthe converter, since less heat energy is required to keep the emitter atthe desired operating temperature. Unfortunately, in practice it hasbeen found that it is not possible to directly employ a low emissivitycollector surface to achieve this end, since the corrosive environmentproduced by the alkali metal or other ionized gas atmosphere used forspace charge neutralization attacks low emissivity surfaces causingsignificant increases in emissivity and a concomitant increase in thethermal radiation absorbed by the collector assembly which results in adecrease in converter efliciency. Therefore, it is one object of thisinvention to teach method and apparatus to eliminate the problemsassociated with achieving near ideal collector emissivities which remainconsistent with time wit-hout suffering attack from the corrosiveenvironment in the interelectrode space of the converter.

It is an additional object of this invention to teach method andapparatus to reduce the amount of heat energy that has to be rejectedfrom the thermionic system at the collector assembly temperature byemploying a system which allows direct rejection from the converter of amajor portion of the emitter thermal radiation. In this type of system,the overall efficiency of the converter is reduced but this loss inefhciency is far offset by the total reduction in overall system weightwhich occurs primarily due to a reduction in the size of the apparatusemployed for heat rejection at the collector assembly temperature.

It is accordingly an object of this invention to teach apparatus andmethod for reducing the mount of emitter generated thermal radiationwhich is absorbed bythe collector assembly of the thermionic converter.

It is an object of this invention to teach method and apparatus forreducing the amount of emitter generated thermal radiation which isabsorbed -by the collector assembly of the thermionic diode or convertercomprising placing a thermal radiation reflecting surface in intimaterelation to the collector structure. l

lt is still a further object of this invention to teach an electroncollector or anode which is transparent to the heat radiated from theemitter but opaque to the electrons being emitted from the emitter,thereby reducing the amount of heat absorbed by the assembly.

It is still a further object of this invention to teach the use of athermal radiation transparent member to protect the thermalradiation-reflecting surface from the high temperature alkali metal orother ionized gas atmosphere used for space charge neutralization .inthe thermionic converter.

lt is still a further object of this invention to teach a thermalradiation transparent and electron opaque anode or collector whichcomprises either a very fine mesh screen or a thin conducting filmplaced by plating or the like on the inner side of the cell wall of athermal radiation transparent anode.

It is still a further object of this invention to teach -a thermionicelectron collector which is transparent to radiated heat lbut opaque toemitted electrons and which includes a thermal reflecting surface whichreflects the emitter thermal radiation back to the emitter.

It is a `further object of this invention to enhance the output ofthermionic converters in arc-mode operation by reflecting the thermalradiation from the collector assembly back through the neutralizationplasma to the emitter since this thermal radiation, primarily theinfrared radiation, contains energy in the wave lengths suitable toexcite cesium or other alkali atoms from the ground state. Such excitedatoms can provide the ions lrequired for space charge neutralization.Further, t-he highly reflective properties of the collector assemblysurface will help to strongly trap thermal radiation of these wavelengths in the interelectrode space.

As more fully explained in a paper entitled An Evaluation Orf TheInRadiator Approach To Nuclear-Thermionic Space Power Systems, by DonaldW. Bell and Arthur I. Chalfant presented at the June l7-2=0, 1963 summermeeting of the American Institute of Aeronautics and Astronautics andpublished by that organization, variations in the thermal emissivity ofone or both of the thermionic electrodes can have a significant effecton the converter efliciency, although the power density is only slightlyaffected. A Ahighly reflective collector not only permits a significantsystem Weight saving but it also prevents significant changes inthermionic converter performance from occurring should the thermalemissivity of the thermionic emitter vary substantially over the systemoperating lifetime. The effect of such variations are illustrated in theaforementioned paper, which shows that the thermionic efficiency changesonly slightly with large changes in emitted thermal emissivity in thelow collector emissivity range of- 0.03 to 0.05. At higher collectoremissivity values, however, the reductions in eiciency with increases inemitter emissivity become more pronounced. T-he effect of suchvariations in emitter emissivity with time on an operating space powersystem would be detrimental since the converters would be operating atoff-design conditions and producing less power than desired. It is,therefore, an object of this invention to reduce or eliminate the effectof thermal emissivity changes in the emitter electrode during the lifeof the converter.

It is still a further object of this invention to reduce changes inthermionic converter performance caused by changesl in electrodeemissivities during the converter lifetime by providing at least oneelectrode with a thermally reflecting surface and protecting thethermally refiectingsurface from corrosion.

It is still a further object of this invention to improve the degree ofionization in a thermionic converter by increasing the reflectivity ofthe collector and thereby increasing the number of times the thermalradiation is reected Ibetween the electrodes.

Other objects and advantages will be apparent from the specification andclaims and from the accompanying drawings which illust-rate anembodiment of the invention.

FIG. 1 is a schematic representation of a thermionic converter using athermal radiation reflecting collector.

FIG. 2 is a schematic representation of a thcrmionic converter usinganother embodiment of this invention and depicts one form of thermalradiation transparent and electron opaque collector.

FIG. 3 is a schematic representation of another form of this inventionand depicts a second form of thermal radiation transparent and electronopaque collector.

FIG. 4 isa schematic representation of a thermionic converter in whichthe collector assembly combines both reflection of emitter thermalradiation using a thermalradiation-transparent, corrosion-resistantmember and an electron-opaque structure for the electron collector whichis substantially transparent to emitter thermal radiation.

Referring to FIG. l, we see thermionic converter or diode which consistsof cathode or emitter 12 and anode or collector assembly 14 withelectrical insulators 16 therebetween. The emitter 12 and the collectorassembly 14 are supported in spaced relation to define interelectrodegap 18 therebetween. Interelectrode gap 18 is evacuated and may befilled with a gas or vapor, such as cesium or other alkali metal but notlimited to these gases. During operation these gases are partiallyionized to prevent or minimize the creation of an electron space charge.

In operation, 4heat is added to emitter 12 to lbring the emitter, whichmay *be made of molybdenum or other refractory material but not limitedspecifically to these materials, to a temperature such as 1400 K., atwhich electrons are emitted -by thermionic emission from surface 20,from which they pass across interelectrode gap 18 to electron collectorand then flow through electrical leads 22 and load 24 and perform usefulwork in passing through lo'ad 24. Due to the proximity between hotemitter 12 to cold collector assembly 14, the thermal radiation,primarily infrared, from emitter 12 is absorbed by collector assembly 14such that conventional cooling means must be used to cool collectorassembly 14. This conventional cooling means may be either fins 2'6 orheat exchanger 28 which includes coolant passed through tubes 30.

This description of the construction and operation of a thermionicconverter is considered to be adequate for the present purpose butgreater detail with respect thereto may be had by referring to U.S.Patent No. 2,980,819 and also to Direct Conversion of Heat toElectricity, edited by Joseph Kaye and John A. Welsh, John Wiley & Sons,Inc., 1960.

It is the object of this invention in one embodiment to increase theefficiency of the thermionic converter by in- Cir creasing thereflection back to the emitter of the thermal radiation from theemitter, which would otherwise be absorbed by the collector assembly.This reduces the amount of emitter generated thermal radiation absorbedby the collector assembly and thereby reduces the amount of coolingrequired to maintain the collector assembly at a desired operatingtemperature and also serves to maintain the emitter at its elevatedtemperature, thereby reducing the amount of heat which needs to be addedthereto by an external source.

As best shown in FIG. l, enhanced thermal radiation reection isaccomplished by providing a low ernissivity surface, such as a highlypolished silver surface, but not limited thereto, 32 behind electroncollector 15 of collector assembly 14 from emitter 12. Due to the actionof low emissivity surface 32, the thermal radiation from emitter 12 isretiected or transmitted by surface 32 of collector assembly 14 back toemitter 12. In this fashion, a large percentage of the emitter generatedthermal radiation which would otherwise be absorbed by ,the collectorassembly 14 is reflected back to the emitter.

To protect thermal radiation reecting surface 32 from the alkali metalvapor or other corrosive environment in interelectrode gap 18, thermalradiation transparent window 34, made of sapphire, or other suitablematerial, is positioned between retiecting surface 32 and electroncollector 15 of collector assembly 14. The aforementioned other suitablematerial from which the thermal radiation transparent window 34 may bemade in addition to aluminum oxide (sapphire), include single crystalberyliurnV oxide (BeO) or single crystal magnesium oxide (MgO) or any ofthe materials listed in the American Institute of Physics Handbook,edition by Dwight E. Gray, second edition, published by McGraw-HillCompany in 1963 and more particularly listed in FIGURE 6c-1 on page 6-47thereof.

The thermal radiation reflecting surface 32 may be made of manymaterials which have suitably low emissivity, for example, the materialslisted in the publication Heat Transmission by William I-I. McAdams,third edition, 1954, published by McGraw-Hill Company and moreparticularly shown on the emissivity table at page 472 thereof, forexample, silver, copper, gold, brass, aluminum or nickel.

Reflecting surface 32 may either be part of collector support plate 36of collector assembly 14 or may be positioned between reector supportplate 36 and window 34, or may be placed onto window 34. Surface 32,support plate 36, window 34 and electron collector 15 are part ofelectron assembly 14.

Preferably electron collector 15 should be made of a semiconductingoxide so as to have a low electrical resistance and be essentiallytransparent to thermal radiation (primarily infrared) and not subject tochemical attack from the corrosive environment in the converterinterelectrode gap 18. Removal of the electron current from the electroncollector 15 may be facilitated by thin wires imbedded in said electroncollector or some similar fashion.

In order to reduce transport losses in the interelectrode gap 18 to aminimum, the emitter and collector assembly 14 are supported in closeproximity by electrical insulating supports 16. Due in part to thisproximity, the micron wave length heat being emitted from emitter 12heats by thermal radiation, the collector assembly 14 and all of thisheat absorbed by the collector must be dissipated as waste heat. Thedissipation of this Waste heat requires the use of a radiator, such as aconventionally nned radiator 26, or y,eat exchanger 2S to be used inconnection with collector assembly 14 to maintain the temperaturethereof relatively low in comparison to the temperature of emitter 12.

It is a further object of this invention to teach a collector assemblywhich is transparent to the heat radiated by the emitter, but opaque tothe electron emission therefrom.

An embodiment of a transparent anode or collector is shown in FIG. 2.Referring to FIG. 2 We see emitter 12' and transparent anode orcollector assembly 14' positioned on opposite sides of interelectrodegap 18. Transparent collector assembly 14 comprises very fine screenelectron collector 130 which is selected to offer small cross-sectionalarea to emitter radiation butto be a good conductor which draws from theinterelectrode plasma high electron currents, and cell wall material132. Screen 130 may be made of any conductor which is insensitive tochemical attack, such as copper but not limited thereto, and befabricated of a mesh suitable for high transmission of radiation and lowelectrical resistivity such as solid frontal area. Cell wall material132 is selected to be transparent to radiation typical of wave lengthsfrom emitter radiation such that after the radiation passes throughscreen anode 130, it will be radiated to space or any convenient heatabsorption means through the transparent cell wall material 132, whichmay be sapphire or other suitable material. The aforementioned othersuittable material from which the thermal radiation transparent cellwall material 132 may be made in addition to aluminum oxide (sapphire),include single crystal berylium oxide (BeO) or single crystal magnesiumoxide (MgO) or any of the materials listed in the American Institute ofPhysics Handbook, edition by Dwight E. Gray, second edition, publishedby McGraw-Hill Company in 1963 and more particularly listed in FIGURE6c-1 on page 6-47 thereof.

Another embodiment of a transparent anode or collector is shown in FIG.3 and is identified as 14". In this FIG. 3 embodiment, collectorassembly 14 includes therrnal radiation transparent cell wall 132',which may be made of any of the materials suggested above for use ascell wall member 132 of FIG. 2 and a thin conducting film 134 used as anelectron collector, which may be made of a semiconducting oxide but notlimited thereto and which is preferably maintained as thin as isconsistent with conducting typical diode currents of about l0 amps/ cm.2from this surface, is placed on the cell walladjacent to emitter 12 andhas electrical leads 22 projecting therefrom. Lead wires 22 are selectedto offer the smallest possible cross-sectional area to emitter radiationand essentially the transparent purpose of cell 132 will not be affectedby either the thin r'ilm placed on the surface thereof or the currentleads 22 connected to the film. In either the FIG. 2 or FIG. 3configurations, cell wall members 132 and 132', respectively, may havean inrared radiation surface such as surface 32 of FIG. l on the side ofcell 1vall members opposite to emitters 12 and 12, respectively.

The advantage of a transparent anode system is that a smaller radiatingsurface is needed to reject Waste heat in space power applicationsbecause with this system a major portion of the W-aste heat is in theform of. thermal radiation which is rejected at the emitter temperaturerather than at the collector assembly temperature and need not bebrought down to the collector assembly temperature prior to rejection.For example, consider a conventional space thermionic power systemoperating at 15% etiiciency with an emitter temperature of l400 K.,collector assembly temperature of 900 K. This means that 15% of the heatinput to the emitter is converted to useful electricity. The remaining85% of heat input is waste energy which must be removed from the system.Rejection of a major portion of this waste heat at the emittertemperature (l400 K.) rather than the collector assembly temperature(900 K.) will significantly reduce the weight of the radiator. It shouldtbe noted, however, that the overall heat input to the emitter wouldhave to be higher since the collector is not reecting heat back to theemitter. However, the weight increase required for the higher heat inputwould be more than offset by the 6 Weight reduction of the radiatorsurface in a nuclear space power application.

Since it can be shown mathematically that the radiator size varies as alower limit inversely with the temperature of the collector raised tothe fourth -power (T4), the weight penalty imposed by the radiator isone of the main contributing factors to the system weight forout-of-pile thermionic space power systems as shown by theaforementioned Bell and Chalfant paper.

The performance of a thermionic power system could be increasedsubstantially by using the construction shown in FIG. 4 in which coppercollecting tins, but not limited thereto, 40 cooperate to form atransparent grid collecv tor and have radiation transparent windows 42positioned therebetween. A highly reflective coating, such as a highlypolished silver surface, is placed behind a transparent window such assapphire 42 at surface 44 to reflect the waste heat back to emitter 12'.Transparent windows 42 serve to protect the highly reflective surface 44from the alkali metal or other corrosive material environment existingin the gap of the converter. Thermal radiation transparent windowmembers 42 may be made from any of the materials which were previouslyenumerated above as tbeing ac-ceptable for use as the thermal radiationtransparent window member 34 of FIGURE 1 and the thermal radiationtransparent cell wall material 132 or 132 of FIGS. 2 aind 3.

With respect to FIG. 4 construction and assuming a collector assemblystructure with 90% transparency and a heat discharge requirement of 30watts/cm.2 at the front of the grid collector surface, the calculatedtemperature drop across a l mm. copper grid is approximately 9.5 K.according to the following formulae:

KAAT Q: e 1) (s.2) (o.1)AT 30 Wartt/Cmk- AT=9..3 Kl. (3)

conv-iA L EC-l-EA This represents the total emissivity for aconventional operating thermionic converter.

...iii m erm-1 8) where econ., is the emissivity of a conventionalconverter, emmor is the total emissivity of a mirror electrode, sc isthe emissivity of the cathode, eAisthe emissivity of the anode.

This represents the total emissivity for a mirror-type surface inthermionic converter operation whichhas been 7 protected from theeffects of the corrosive cesium environment.

Then the total grid-mirror collector emissivity assuming 90%transparency is:

emirror) eoonv) 0.9( 0.0268) -l- 0.1(0.935) o.oss47 The grid-mirrorcollector thus provides a reduction in overall emissivity byapproximately a yfactor of three, which. should result in an approximateincrease in eficiency of one or two points with a reduction in spacepower system weight of approximately 1-3 lb./kwe. as shown by theaforementioned Bell and Chalfant paper.

It would be possible to use a single layer of material on theinfrared-reflecting surface if the material were infrared-transparentand an electron collector with suitable work function.

It is to be understood that the invention is not limited to' thespecific embodiment herein illustrated and described but may be used inother ways without departure from its spirit as defined by the followingclaims.

We claim:

1. In a thermionic converter, an electron emitter adapted to be heatedto' a temperature to emit electrons and generate thermal radiation, athermal-radiation-transparent and electron-opaque collector assembly,means to support said emitter and collector assembly in spaced relationto form an inter-electrode gap therebetween, and means electricallyconnecting said collector assembly to said emitter.

2. Apparatus according to claim 1 and wherein said collector assemblyincludes an electron collector member comprising a fine mesh screenpresenting minimal -cross-sectional area to said radiation and a goodelectrical conducting media to said electrons.

3. Apparatus according to claim 2 and including a radiation transparent,corrosion resistant member behind said screen.

4. Apparatus according to claim 3 wherein said member is made ofsapphire.

5. Apparatus according to claim 1 wherein said collector assembly is athin conducting film placed on a radiation transparent, corrosionresistant member.

6. Apparatus according to claim 1 wherein said collector assembly is aradiation transparent corrosion resistant member with a thin conductinglm placed thereon on the surface adjacent said emitter and including ahighly radiation reflecting surface adjacent said corrosion resistantmember on the side opposite said emitter.

7. In a thermionic converter, a cathode, an anode, means enveloping saidcathode and anode in a controlled atmosphere, means supporting saidcathode and anode in spaced relation to form an interelectrode gaptherebetween, said anode comprising a fine mesh screen of smallcrossesectional area and good electrical conductivity positionedadjacent said cathode and a radiation transparent corrosion resistantmember on the opposite side of said screen `from said cathode and ahighly heat reflective surface on the opposite side of said corrosionresistant member from said cathode.

8. In a thermionic converter, an electron emitter adapted to be heatedto a temperature to emit electrons vand generate thermal radiation, acollector assembly,

means to support said emitter and collector assembly in spaced relationto form an -interelectrode gap therebetween, means electrically-connecting said emitter and collector assembly, said collector assemblyincluding an electron collector member communicating with saidinterelectrode gap and a collector support plate attached to andsupporting said electron collecting member, a thermal radiationreflecting surface positioned between said collector support plate andsaid electron collecting member and presenting athermal-radiation-reflecting-surface toward said emitter, and athermal-radiation-transparent, corrosion-resistant memberpositionedbetween said reflecting surface and said electron collectormember so that the thermal radiation generated in said emitter is`reflected from said reecting surface back to said emitter to reduce theamount of emitter generated thermal radiation absorbed by said collectorassembly.

9. In a thermionic converter, an electron emitter adapted to be heatedto a temperature to emit electrons and generate thermal radiation, acollector assembly, means to support said emitter and collector assemblyin spaced relation to form an interelectrode gap therebetween, meanselectrically connecting said collector assembly and said emitter, acooling structure, said collector assembly including a plurality ofelectron collecting fins joined to form a grid-type collector in spacedrelation to and exposed to said emitter and with said fins attached tosaid cooling structure, said collector assembly further including aplurality of thermal-radiation-transparent members joined to andextending between said fins and cooperating therewith to dei-inc athermal-radiation-transparent, electron-opaque grid anode, and saidmembers having a plurality of thermal radiation reecting `surfacespositioned on the side thereof away from said emitter and -facing saidemitter so that the thermal radiation from said emitter passes throughsaid radiation-transparent members to said radiation reflecting surfacesfrom which the thermal radiation is reflected back to the emitter,thereby reducing the amount of emitter-generated thermal radiationabsorbed by the collector assembly.

10. In a thermionic converter, `an electron emitter, an electroncollector assembly including an electron collector, means supportingsaid emitter and collector assembly in electrical isolation, meansenveloping said emitter and electron collector, said collector assemblyincluding means to prevent the absorption of emitter generated thermalradiation by the collector assembly.

11. Apparatus as in claim 10 and including a thermalradiation-transparent member between said electron collector and saidreflection surface.

12. Apparatus as in claim 10 wherein said electron collector assembly isessentially transparent to thermal radiation.

13. In a thermionic converter, an electron emitter, an electroncollector, means to support said emitter and collector in spacedrelation to establish an interelectrode gap therebetween collectorsupport means, a highly polished silver surface behind said collectorfrom said emitter, and a protective sapphire window between saidcollector and said surface.

14. In a thermionic converter, lan electron emitter adapted to be heatedto a temperature to emit electrons and generate thermal radiation, athermal radiation nonabsorptive electron collector assembly including anelectron collector, means supporting said emitter and collector assemblyin electrical isolation and in spaced relation to form an interelectrodegap therebetween, means enveloping said emitter and electron collector.

References Cited UNITED STATES PATENTS 2,319,912 5/1943 Anderson 313-473,201,618 8/1965 Coleman y310--4 3,243,612 3/1966 Lyczko 310-4 3,248,5774/1966 Hoh 310-4 FOREIGN PATENTS 919,148 2/ 1963 Great Britain.

OTHER REFERENCES Gabor, D.: A New Thermionic Generator. In nature, vol.189, Mar. 18, 1961, pp. 868-872.

MILTON O. HIRSHFIELD, Primary Examiner,

D. F. DUGGAN, I. W. GIBBS, Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No $5376,4"5'7 April 2 1968 Russell. G Meyerand Jr et al certified that. errorappears in the above numbered pathereb a It 1s y on and that the saidLetters Patent should read as ent requiring correcti corrected below.

Column 8 lines 38 to 40 cancel "Apparatus as in claim l0 and including athermal radiationtransparent member between said electron collector andsaid reflection surface" and insert In a thermionic converter, anelectron emitter, an electron collector assembly including an electroncollector, means supporting said emitter and collector assembly inelectrical isolation, means enveloping said emitter and electroncollector, and means operatively associated with said emitter andelectron collector` to reduce the total emitter generated thermalradiation absorbed by the collector assembly including a thermalradiation reflection surface operatively associated with said collectorassembly and a thermal radiation transparent member between saidelectron collector and said reflection surface Signed and sealed this12th day of August 1969.

(SEAL) Attest:

EDWARD M; PLETCHERJR. WXLLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

1. IN A THERMIONIC CONVERTER, AN ELECTRON EMITTER ADAPTED TO BE HEATEDTO A TEMPERATURE TO EMIT ELECTRONS AND GENERATE THERMAL RADIATION, ATHERMAL-RADIATION-TRANSPARENT AND EELCTRON-OPAQUE COLLECTOR ASSEMBLY,MEANS TO SUPPORT SAID EMITTER AND COLLECTOR ASSEMBLY IN SPACED RELATIONTO FORM AN INTERELECTRODE GAP THEREBETWEEN, AND MEANS ELECTRICALLYCONNECTING SAID COLLECTOR ASSEMBLY TO SAID EMITTER.